1. Field of the Invention
This invention relates to a video cipher processing system for use in TV broadcasting, and more particularly to improvements in a system for use in a pay TV wireless broadcasting in which a broadcasting system station transmits ciphered TV signals obtained by inverting video signals by means of an encoder. The ciphered TV signals are decoded by a subscriber's decoder to be reproduced in the TV receiver.
A wireless pay TV system has been adopted and developed in response to a public desire for better programs, even if for a fee, because general commercial broadcasting programs are filled with advertisements and are held by some to contain vulgar contents due to various social are economical restrictions.
Such a system is expected to be more useful in the future because it would have the advantage that the time and expense which is required for spreading cables can be saved by a wireless system and that the number of subscribers per system is not subject to any restriction.
Generally in such wireless pay TV systems, video signals and voice ignals are ciphered so as to be available to subscribers only, excluding others. A conventional example of those systems is shown in FIG. 1. Video signals at the transmitter applied to an encoder input terminal 1 are subjected to controlled polarity inversion by a video signal polarity inverter 3 to be sent therefrom to an output terminal 2. At this time, the video signal polarity inverter 3 is controlled by a random number signal generator 4. That is, the video signal polarity inverter 3 produces an output but selectively inverting the polarity of the applied video signals in response to a logical value signal from the random number signal generator 4, which value varies at random. A decoder is arranged to decode the ciphered video signals in accordance with a previously prepared key code signal so as, to reproduce a visible picture so that only a subscriber having such a decoder can look at and listen to the program.
However, in such a conventional ciphering system for the picture signals, there have occurred discrepancies between the original video signals and the reproduced video signals, due to the differences between the characteristics (linearity, for example) of the parts used in the encoder and the decoder, respectively, so as to cause flickers in the screen arising from irregular luminance and hue variation. Such drawbacks might be eliminated by making the parts used in the encoder and the decoder have more controllably the same characteristics, or by keeping a precise adjustment and maintenance thereof.
However, such means are not preferable from a manufacturing and economical point of view.
In order to reduce such occurrence of flickers, there has been proposed another system for processing ciphers in which by detecting the average tone variation of the video signals they are submitted to polarity inversion only upon occurrence of significant average tone variation i.e. when the brightness of the screen varies due to change of scenes.
However, video signals ciphered and transmitted by the above-mentioned system also have the possibility of being reproduced by detecting the average tone variation thereof without decoding ciphers by means of a decoder, and thus, there has arisen the problem that the deciphering manner cannot be kept secret.